1. Field of the Invention
The present invention relates generally to tensioners used with chain drives in automotive timing and power transmission applications. In particular, the present invention is related to a ratcheting tensioner system.
2. Description of Related Art
A tensioning device, such as a mechanical tensioner, is used as a control device for a power transmission chain, or similar power transmission devices, as the chain travels between a plurality of sprockets. In this device, the chain transmits power from a driving sprocket to a driven sprocket, so that part of the chain is slack and part of the chain is tight. Generally, it is important to impart and maintain a certain degree of tension in the chain to prevent noise, slippage, or the unmeshing of teeth in the case of a toothed chain. Prevention of such slippage is particularly important in the case of a chain-driven camshaft in an internal combustion engine because jumping of teeth will throw off the camshaft timing, possibly causing damage or rendering the engine inoperative.
However, in the harsh environment of an internal combustion engine, various factors can cause fluctuations in the chain tension. For instance, wide variations in temperature and thermal expansion coefficients among the various parts of the engine can cause the chain tension to vary between excessively high or low levels. During prolonged use, wear to the components of the power transmission system can cause a decrease in chain tension. In addition, camshaft and crankshaft induced torsional vibrations cause considerable variations in chain tensions. Reverse rotation of an engine, occurring for example in stopping or in failed attempts at starting, can also cause fluctuations in chain tension. For these reasons, a mechanism is desired to remove excessive tensioning forces on the tight side of the chain and to ensure the necessary tension on the slack side of the chain.
Mechanical blade and hydraulic tensioners have been used in the past to apply tension to chains. Using mechanical tensioners is a common method of maintaining proper chain tension. In general, these mechanisms employ a lever arm that pushes against the chain on the slack side of the power transmission system. This lever arm must push toward the chain, tightening the chain when the chain is slack, and must be rigid when the chain tightens.
FIG. 1 shows a hydraulic tensioner with a flexible tensioner arm with a blade spring as disclosed in U.S. Pat. No. 6,447,414. The hydraulic tensioner is installed adjacent to a chain 10, which is shown schematically. The chain 10 includes two strands, 12, 14, which each contact opposite sides of the tensioner. The tensioning system includes an arm 16 and a hydraulic tensioner 20 in a hydraulic tensioner housing 40. The arm 16 applies tension along the upper chain strand 12. The second chain strand 14 travels along a stationary arm 42 or path at the bottom of the tensioner housing.
One end 17 of the arm 16 is pivotally attached to a support 22, which forms part of the tensioner housing. The center portion of the arm 16 is supported by the upper portion 24 of the piston 26 of the hydraulic tensioner 20. The free end 19 of the arm 16 is supported by another housing support 28. The arm 16 is positioned so that its upper side 32 is against the underside 34 of the chain strand 12. The bottom side 36 of the arm 16 is positioned against the upper side 24 of the tensioner piston 26. One end 17 of the arm 16 has a single pivot about a point 38, while the other end 19 is free and can slide along one of the housing supports 28. The movement of the crankshaft sprocket 44 forces the chain 10 and the respective strands 12 and 14 into motion. The chain strand 12 wraps about a balance shaft sprocket 46.
FIG. 2 shows a mechanical tensioner as disclosed in U.S. Pat. No. 6,849,015. The tensioner is positioned to control a tight strand 64 and a slack strand 65 of a timing chain 66. An arm 54 carries a shoe 53 positioned outside the slack strand 65 of the timing chain 66. The tensioner arm 54 takes up the slack and applies tension to the chain if moved in the direction of arrow 59. The arm 54 and shoe 53 are preferably attached by a fixed pin 58 to a tensioner pivot arm 51. The tensioner pivot arm 51 has a pivot feature at pivot 52. Tensioning occurs when a force is applied at point 55 in the direction of arrow 56. Rotational force in the direction of arrow 56 results from an extension spring 57 attached at point 55. The type of spring used to apply this rotational force includes a torsional spring, a compression spring, a tension spring, or a hydraulic tensioner.
Excessive reverse rotation of the tensioner pivot arm 51 is limited by adding a ratchet feature, such as ratchet teeth 61, which interface with a pawl 63. Tightening and slackening of the chain is caused by the effect on its environment (for example, the engine block, cylinder head, water, oil, etc.) by temperature changes (for example, a hot engine or a cold engine). To accommodate this tightening and slackening, the assembly requires some amount of backlash. As an example, the backlash is illustrated as a slotted or oversized hole in the pawl 63 at its pivot pin with a spring force applied to the pawl 63 by extension spring 60 in the direction of arrow 62 and against the ratchet teeth 61. The ratchet teeth 61, pawl interface, and pin size, etc. are determined in accordance with chain loads as affected by the various link arms and/or moment arms and suitable ratchet increments.
When the attachment of the arm 54 and shoe 53 to the tensioner pivot arm 51 at point 58 allows the arm 54 and the shoe 53 to pivot as illustrated, the shoe 53 aligns itself to the chain 66, balancing the loads of its two halves against friction while yielding to chain motion (i.e., chordal fall, chain jerk, chain wave, etc.).